Note: Healthy was launched about 5 months ago but I've only just got around to writing this post.
We were also challenged to create an application using open data that gets people to eat healthier. We came up with Healthy, a calculator that tells you "the time to burn" of a particular food.
It's built on top of Node.js and uses Socket.io to communicate with the browser in real time. This means no page refreshing, no Ajax calls and no latency. We use an API to collect the information about the food the user has requested and then use some algorithms to figure out how much time doing a particular excercise is needed to burn off the calories.
I worked mainly on the algorithms to extract the information from the API and also wrote a large amount of the main JS for the Node.js web server and Socket.io integrations.